New Laser Power Measurement Technique Improves Accuracy

Written by: James Wilkey Based on: Measuring Laser Beam Welding Power Using the Force of Light; AWS Welding Journal; March 2016; Paul Williams, Jeffrey Sowards, and Brian Simonds

How do you measure the force of light? It’s something scientists have discussed since the 1700s. Today, that question is being answered with a new process that stands to revolutionize laser welding applications. Featured in this month’s Welding Journal, the new process uses a specially-made mirror that measures laser power while reflecting light to the weld.

Laser welding is a process that requires precision. And yet, measuring the power of a laser in real-time while welding is anything but precise. Traditional means of measuring laser strength rely on temperature data. The laser’s unfocused beam is directed onto a thermal power reader, a device coated with a highly-efficient, light-absorbing substance that allows the meter to measure the heat delivered by the laser with significant accuracy. Unfortunately, if the laser is being aimed at the thermal power meter, it isn’t being aimed at the weld. Thus, determining reliable settings for effective laser welding usually results in a trade-off between extremely accurate laser beam measurements taken when the laser is not welding, or dramatically less accurate data taken from the laser while the beam is welding in real-time. The limited data gathered with current methods of measuring laser power are sufficient for many industries. However, they fall short of the rigid standards and requirements of several fields, such as the aerospace industry. Consequently, it is those fields that stand to benefit the most from the precision-power delivery of a laser.

In response to the demand for more accurate, real-time laser data without the tradeoffs, the National Institute of Standards and Technology worked to develop a new means of measuring laser welding power. This was accomplished by incorporating a mirror that does two things. First, the mirror measures the force with which the laser’s photons, or light particles, hit the surface of the mirror. The mirror is able to accomplish this through the use of sensitive instruments that are attached to its surface. These sensitive instruments measure the momentum with which the photons strike the mirror. It might seem surprising that light particles would have any force, but a laser beam is actually a dense collection of tiny light particles being rapidly fired at a small space. Imagine a machine spitting out trillions of tiny, high-speed tennis balls at a single target: this is essentially a laser beam. Second, the highly reflective mirror redirects the beam’s photons toward the workpiece. By reflecting light to measure the force with which the beam strikes, rather than absorbing the beam to measure heat, data can be collected from the laser without interrupting or diminishing the welding process.

The new process has resulted in never-before seen accuracy. The tiny impacts made by the photons on the mirror are conveniently proportional to the laser’s actual power output at the weld, allowing for measurements with an unprecedented 1 to 2 percent accuracy rating. All of this is done while the weld is in progress. Such accurate, real-time data will allow for the development of more efficient and effective standards for setting the parameters of laser welding applications.

To read more about this new process, including a walkthrough of the team’s full experiment for testing this new method, check out “Measuring Laser Beam Welding Power with the Force of Light” in the March 2016 issue of the Welding Journal, free with your AWS membership.

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